Restructure the README around the reader's path

[sturimcode]

Full audit of the README against the code, plus a restructure around the reader's path instead of the order sections were written in. No code changes.

Fixes

• The tagline said "macOS and Windows" while the doc itself has a Headless Linux section; it now names all three platforms.
• --history and --headless existed in the CLI but not in the README. Both documented; --version, --config, and --db get a pointer to --help.
• The systemd recipe used --headless without the flag being documented anywhere.

Restructure

• "How Garmin login works" moved from before Install to after "How it works": new users no longer read Cloudflare lore before the install command.
• Install split into macOS / Windows / Linux subsections, uv first on all of them.
• Automatic sync now opens platform-neutral, then per-OS bullets (Launch Agent / Scheduled Task / none, use systemd), matching what platform_support actually does.
• The standalone Updating section is gone; updating was explained three times, now once under Usage.
• Credential storage merged into Security as a per-platform list of where secrets live.
• Known quirks reordered by blast radius: app-must-process-weigh-in first, rounding trivia last.
• Usage flags grouped by task; Stability column dropped from the targets table (every row said Stable).

Left alone on purpose

• ~/.garmin-sync/ naming (renaming breaks installs).
• Tests section: making it a real Development section needs a decision on the contributor setup, since pytest is not declared in pyproject.

Verified against code: weekly update check (shared.py), diagram filenames, log paths, and the generic platform layer's no-agent behavior.

🤖 Generated with Claude Code